Don't allow the filesize of the site to be calculated in multiple thread at once.

Currently if the sites size is being calculated and you reload the page before the calculation has finished then another size calculation will be kicked off, on large sites this can cause issues as several requests can end up open, all calculating the size of the site.

This commit fixes that by ensuring that at a max you only get one size calculation per hour.

Fixes #229
Merge pull request #229 from humanmade/single-calculating-thread

Only one filesize calculating thread per hour.